1. Field of the Invention
Current beneficiation practices for phosphate ores commonly utilize flotation as the principal step for separating phosphates from gangue minerals. The "Crago" or "double float" froth flotation process is commercially used for beneficiating such phosphate ores in which silica and silicate minerals are the predominant constituents of gangues. That process consists of conditioning the material with fatty acids and fuel oil, flotation of phosphate minerals, deoiling with sulfuric acid to remove the reagents, and refloating with amine reagents to remove the siliceous gangue which either floated or was trapped in the rougher float (U.S. Pat. No. 2,293,640).
However, many phosphate ores of potential commercial value contain carbonate gangue mineral matter in addition to siliceous minerals. These carbonate mineral impurities include dolomite, calcite, dolomitic limestone, seashell, and other less common carbonate-type minerals. Such mineral impurities not only dilute the P.sub.2 O.sub.5 content of the phosphate ore concentrate, but also may interfere in subsequent chemical processing thereof. For example, carbonate minerals present in phosphate ore concentrates used to produce phosphoric acid, superphosphate, or triple superphosphate consume acid reagents in the acidulation steps without providing additional fertilizer values. They aggravate foam formation in the reactor vessel. The presence of dolomite or dolomitic limestone in the phosphate ore concentrate is particularly important because relatively small amounts of magnesium (i.e.&gt;1 percent MgO) may cause technical problems in current wet-process acid chemical plants due to increased viscosity of acid, increased defoamer usage, scale and sludge formation, and possibly difficulty in maintaining a standard diammonium phosphate (DAP) fertilizer grade.
The "double float" process has generally been ineffective for beneficiating such high carbonate phosphate ores. Collectors such as fatty acids used for phosphate minerals generally float carbonate minerals as well, and it is only by the employment of suitable selective depressants of either phosphate or carbonate gangue that adequate selectivity can be obtained.
2. Description of the Prior Art
Methods of beneficiating phosphate ores containing carbonate and siliceous gangue materials are described in the following U.S. Pat. Nos.: 3,259,242, Snow, July 1966; 3,462,016, Bushell et al, August 1969; 3,462,017, Bushell et al, August 1969; 3,807,556, Johnston, May 1974; 4,144,969, Snow, March 1979; 4,189,103, Lawver et al, February 1980; 4,287,053, Lehr et al, September 1981; 4,317,715, Kintikka et al, March 1982; 4,364,824, Snow, Dec. 21, 1982; and 4,372,843, Lawver et al., February 1983. Each of these patents deals with processes that differ from the present invention, as for example: Snow, in U.S. Pat. No. 3,259,242 supra describes the beneficiation of a macro-crystalline apatite-calcite mixture which utilizes a cationic flotation to collect apatite as a float concentrate. Bushell et al, in U.S. Pat. No. 3,462,016 supra, teaches the use of H.sub.3 PO.sub.4 and Bushell et al in U.S. Pat. No. 3,462,017 supra teach the use of NH.sub.4 H.sub.2 PO.sub.4, both as phosphate mineral depressants for beneficiating carbonate-bearing phosphate rock. Johnston, in U.S. Pat. No. 3,807,556 supra further teaches the use of soluble sulfate salts such as Na.sub.2 SO.sub.4 or (NH.sub.4).sub.2 SO.sub.4 to reduce the loss of soluble phosphate in the processes of U.S. Pat. Nos. 3,462,016 and 3,462,017 supra.
The use of an apatite-collecting cationic reagent and a liquid hydrocarbon is described in U.S. Pat. No. 4,144,969 and 4,189,103 supra. These patents describe a phosphate ore beneficiating process in which the deslimed ore is first subjected to a "double float" froth flotation as described in U.S. Pat. No. 2,293,640, Crago, Aug. 18, 1942, to remove siliceous gangue. The float product containing apatite with dolomite impurities is then conditioned with a cationic agent and a liquid hydrocarbon to concentrate apatite as float.
Hintikka et al, in U.S. Pat. No. 4,317,715 supra describe two alternative processes to separate phosphate minerals from carbonate minerals after phosphate and carbonate minerals are concentrated from silicates; (1) the combined phosphate-carbonate concentrate is treated with a base, whereafter phosphate mineral is floated with a cationic collector, or (2) the combined phosphate-carbonate concentrate is treated with SO.sub.2 or CO.sub.2 to recover phosphate as a float.
U.S. Pat. Nos. 4,364,824 and 2,372,843 supra, describe a flotation process for removing carbonate mineral impurities. The process employs salts of sulfonated linear fatty acids as a carbonate mineral collector and a phosphate depressant, such as polyphosphate, metaphosphate, pyrophosphate or orthophosphate, to remove carbonate minerals in the overflow and to collect the phosphate concentrate in the underflow.
Lehr et al in U.S. Pat. No. 4,287,053, assigned to the assignee of the present invention, describe a phosphate ore beneficiating process in which phosphate ores containing carbonate mineral impurities are subjected to froth flotation in the presence of phosphate depressants, said depressants comprising alkyl diphosphonic acids, and a carbonate collector, said collector comprising fatty acids, removing the separated carbonate minerals from the overflow, and recovering the phosphate value in the underflow.
Lehr et al supra also teach that, in practicing their flotation method, the phosphate ores containing siliceous gangues preferably is first beneficiated by conventional techniques, such as the "double float" or "Crago" froth flotation process, to concentrate the phosphate values. Subsequently, the phosphate concentrate containing carbonate mineral impurities is subjected to a final stage of flotation employing the method of the said invention to remove carbonate minerals and produce a salable product. See for example, U.S. Pat. No. 4,287,053 supra, column 2, lines 16-24.
This method of Lehr et al works well when dolomite is unweathered and evenly distributed in the different size fractions of the flotation feed, or when dolomite content is particularly high in the fine-size fractions (for example, -100 +150 mesh, -150 +200 mesh, -200 +270 mesh, etc.). However, their method is fraught with difficulties when applied to some phosphate ore containing coarse, weathered dolomite in the flotation feed, such as that from central and south Florida. A phosphate concentrate having satisfactory P.sub.2 O.sub.5 grade and MgO content cannot be obtained with reasonable recovery efficiency using the conventional Crago process, followed by Lehr's diphosphonic acid depressant process. The coarse, weathered dolomite carried over from the phosphate rougher flotation of the Crago process was difficult to float in the carbonate flotation stage using Lehr's work. Therefore, the present invention is intended to beneficiate this and other similar types of phosphate ores which contain coarse, weathered dolomite in the flotation feed.